We study high-order harmonic generation (HHG) from oriented asymmetric molecule ${\mathrm{HeH}}^{+}$ in strong orthogonally polarized two-color (OTC) laser fields numerically and analytically. We focus on the time-resolved dynamics of HHG from the asymmetric system. The time-frequency distribution of the calculated HHG spectrum shows a strong dependence on the molecular orientation. In particular, when the molecular axis is parallel to the fundamental field of OTC which is stronger than the second-harmonic field of OTC in our simulations, the HHG shows a strong emission in one half cycle and is weak in the other half. This phenomenon is somewhat similar to an optical diode and can be attributed to the permanent-dipole effect of ${\mathrm{HeH}}^{+}$, which is sensitive to the molecular orientation. It suggests a method for controlling the motion of an electron inside an asymmetric molecule and obtaining the desired attosecond pulse train with OTC field. Using a strong-field model, the intrinsic relationship between odd-order and even-order harmonics in OTC field and the structure of the asymmetric molecule is also discussed.

Light is known to exhibit quantum uncertainty in terms of its amplitude, phase, and polarization. However, quantum uncertainty related to coherence, which is also a fundamental physical property of light, remains to be considered. Here, we formulate and explore the concept of quantum optical coherence uncertainty. We focus on the first-order coherence of the simplest possible light field, a purely monochromatic plane wave, which is classically completely stable. Starting from a scalar treatment, we show that the field displays zero coherence uncertainty only for a number state. We then proceed to the vectorial regime and establish that any state leads to coherence fluctuations, governed by a set of uncertainty relations depending on the polarization state and space-time points. Our work thus provides fundamental insights into the quantum character of optical coherence, with potential benefits in applications using highly sensitive interferometric and polarimetric techniques.

One of the crucial generic techniques for quantum computation is amplitude encoding. Although several approaches have been proposed, each of them often requires exponential classical-computational cost or an oracle whose explicit construction is not provided. Given the growing demands for practical quantum computation, we develop moderately specialized encoding techniques that generate an arbitrary linear combination of localized complex functions. We demonstrate that $n_{\mathrm{loc}}$ discrete Lorentzian functions as an expansion basis set lead to efficient probabilistic encoding, whose computational time is $O(max(n_{\mathrm{loc}}^{2}log{n}_{\mathrm{loc}},n_{\mathrm{loc}}^{2}log{n}_q,n_q))$ for $n_q$ data qubits equipped with ${log}_2{n}_{\mathrm{loc}}$ ancillae. Furthermore, amplitude amplification in combination with amplitude reduction renders it deterministic analytically with controllable errors, and the computational time is reduced to $O(max(n_{\mathrm{loc}}^{3/2}log{n}_{\mathrm{loc}},n_{\mathrm{loc}}^{3/2}log{n}_q,n_q)).$ We estimate required resources for applying our scheme to quantum chemistry in real space. We also show the results on real superconducting quantum computers to confirm the validity of our techniques.

We introduce an even-parity precession protocol that can detect the nonclassicality of some quantum states using only measurements of a uniformly precessing variable at different points in time. Depending on the system under study, the protocol may detect the Wigner negativity of a single quantum harmonic oscillator or of a single spin $j\ge 2$; the non-Gaussian entanglement of two harmonic oscillators; or genuine multipartite entanglement of a spin ensemble, whose total spin is an integer. Unlike in other nonclassicality tests, simultaneous or sequential measurements are not required. Our protocol can also detect states that commute with the parity operator, which were missed by similar protocols built from Tsirelson's original precession protocol. This work also closes a long-standing gap by showing the possibility of detecting the Greenberger-Horne-Zeilinger entanglement of an even number of qubits using only collective spin measurements.

We propose controllable, robust, and fast entangled-state transmission schemes with the band-gap state of the non-Hermitian trimer Su-Schrieffer-Heeger (SSH3) chain. The non-Hermiticity is induced by the periodic on-site imaginary potential on each unit cell as gain and loss of the SSH3 chain. We realize topologically protected bidirectional entangled-state transmission in an even-sized chain and unidirectional entangled-state transmission in an odd-sized chain and analyze the effect of the imaginary potential on the transmission performance. It is found that the success of the state transmission mainly depends on the on-site imaginary potential of the output port and is independent of the input port. Moreover, the opportune imaginary potential not only can be used to open or close the transmission channel but also can accelerate the transmission without breaking the robustness. Additionally, we discuss the average fidelity with process assessment and experimental feasibility of the present transmission schemes. The controllable, robust, and fast entangled-state transmission schemes enrich the research on quantum information processing with non-Hermitian quantum systems.

We report the design and theoretical analysis of Wannier-Stark ladders of diamond Lamb wave resonators that feature mechanical compression modes with ultralow damping rates and host spin qubits with excellent optical and spin properties. The degree of localization in the mechanical Wannier-Stark ladder, which is determined by the ratio of coupling rate to frequency spacing between adjacent resonators, sets the effective range of phonon-mediated coupling between spin qubits. Three nearest-neighbor coupling schemes with distinct geometric configurations and a large range of coupling rates have been developed and analyzed. Additional analysis on the effects of disorder indicates that the proposed Wannier-Stark ladder can be robust against realistic experimental imperfections. The development of quantum networks of spin qubits with long-range connectivity can open the door to the implementation of newly developed quantum low-density parity-check codes in a solid-state system.

The structure of the low-lying rotational levels of the $X^{+}{}^2\mathrm{\Sigma }_g$ $(v^{+}=0)$ vibronic ground state of para-${\mathrm{H}}_2^{+}$ has been determined by combining frequency-comb calibrated continuous-wave high-resolution laser spectroscopy of $n\mathrm{f}$ Rydberg series in the range of principal quantum number $n$ between 28 and 115 and Rydberg-series extrapolation using multichannel-quantum-defect theory (MQDT). The use of accurate quantum-defect parameters obtained from new ab initio calculations enabled the experimental determination of the pure rotational term values of the $N^{+}=2$, 4, and 6 rotational levels of ${\mathrm{H}}_2^{+}$ with sub-MHz accuracy [174.236 744 6(77), 575.455 632 5(86), and 1191.385 571(240) ${\mathrm{cm}}^{-1}$, respectively], and of the corresponding spin-rotational coupling constants with an accuracy of better than 100 kHz [42.21(4), 41.26(8), and 40.04(8) MHz, respectively]. These values are in agreement with the results of first-principles calculations that include high-order relativistic and quantum-electrodynamics corrections to the level energies. To reach the reported accuracy in the Rydberg-series extrapolation, it was necessary to correct for artificial level shifts arising in the MQDT calculations in the vicinity of local perturbations of high-$n$ Rydberg states with a $v^{+}=0{\mathrm{H}}_2^{+}$ ion core caused by low-$n$ core-excited Rydberg states, and resulting from approximations in the treatment of the Rydberg-electron energy in the interacting channels.

The Heisenberg scaling is an ultimate precision limit of parameter estimation allowed by the principles of quantum mechanics, with no counterpart in the classical realm, and has been a long-pursued goal in quantum metrology. It has been known that interactions between the probes can help reach the Heisenberg scaling without entanglement. In this paper, we show that interactions between the probes and the additional dimensions of an ancillary system may also increase the precision of parameter estimation to surpass the standard quantum limit and attain the Heisenberg scaling without entanglement, if the measurement scheme is properly designed. The quantum Fisher information exhibits periodic patterns over the evolution time, implying the existence of optimal time points for measurements that can maximize the quantum Fisher information. By implementing optimizations over the Hamiltonian, the initial states of the probes and the ancillary system, the interaction strength, and the time points for measurements, our protocol achieves the Heisenberg scaling for the parameter of the probe Hamiltonian, in terms of both evolution time and probe number. Our protocol features two aspects: (i) the Heisenberg scaling can be achieved by a product state of the probes and (ii) mere local measurement on the ancilla is sufficient, both of which reduce the quantum resources and the implementation complexity to achieve the Heisenberg scaling. The paper is concluded by the investigation of the effects of noise on this protocol.

We study the effect of the magnetic-field-induced transition (MIT) from the magnetic sublevels $2p^{5}3s{}^{3}P_2$, $M=0,\pm 1$ on linear polarization of the entire $2p^{5}3s{}^{3}P_2\to 2p^6{}^{1}S_0$ emission line following the electron-impact excitation of Ne-like ions based on the density-matrix method for describing polarized radiative emission in an external magnetic field. It is found that the MIT has a significant effect on the polarization properties of the line, which is more prominent at stronger magnetic-field strengths and higher impact electron energies. In addition, such an effect is particularly remarkable for light ions due to the fact that the MIT channel dominates in competition with the intrinsic magnetic-quadrupole channel. These findings highlight the potential of polarization measurements for diagnosing magnetic fields in astrophysical and laboratory plasmas.

With recent advances in simulating quantum phenomena in cold atoms, the higher-rank spin tensor Hall effect was discovered in larger spin systems with spin-tensor-momentum coupling, which is an extension of the celebrated spin Hall effects in larger spins. Previously, it has been proposed that a two-dimensional electron gas with Rashba spin-orbit coupling can generate dissipationless transverse spin current, namely the spin Hall effect. However, later work showed that the spin current is canceled by vertex correction, which was subsequently proven by a cancellation theorem that does not depend on any assumptions related to the scattering mechanism, the strength of spin-orbit coupling, or the Fermi energy. While the recent proposal demonstrates a universal intrinsic spin-tensor Hall conductivity, it is unclear if it vanishes similarly to the spin Hall effect. In this work, we address this critical problem and show that the rank-2 spin-tensor current can be divergent by considering the contributions of both interbranch and intrabranch transitions, which resembles the quantum Hall effect in some sense. So the universal spin-tensor Hall effect cannot be observed in a system with finite size. However, we further show that there is an observable nonzero resonance of spin-tensor Hall conductivity as the Landau levels cross under the magnetic field. Our work reveals interesting conductivity properties of larger-spin systems and will provide valuable guidance for experimental explorations of higher-rank spin-tensor Hall effects, as well as their potential device applications.

Recent experiments demonstrate highly tunable nonreciprocal coupling between levitated nanoparticles due to optical binding [J. Rieser et al., Science 377, 987 (2022)]. In view of recent experiments cooling nanoparticles to the quantum regime, we here develop the quantum theory of small dielectric objects interacting via the forces and torques induced by scattered tweezer photons. The interaction is fundamentally non-Hermitian and accompanied by correlated quantum noise. We present the corresponding Markovian quantum master equation and show how to reach nonreciprocal and unidirectional coupling. Our work provides the theoretical tools for exploring and exploiting the rich quantum physics of nonreciprocally coupled nanoparticle arrays.

In this work, we propose an all-optical stroboscopic scheme to simulate an open quantum system. By incorporating the tritter, consisting of a group of beam splitters, we find the emergence of spontaneous antiphase synchronization in the steady state. To better understand the synchronization and entanglement properties within the system, we utilize the relative error measure and find the distribution of logarithmic negativity in parameter space shows similar structures with the results of synchronization measure. Finally, we derive the adjoint master equation corresponding to the system when the synchronization condition is satisfied and explain the existence of oscillations. In addition, we explore the effect of non-Markovianity on synchronization, and we find that it only slows down the time for the system to reach the steady state but does not change the synchronization properties of the steady state. Our work provides a promising scheme for experimental studies focused on synchronization and other nonequilibrium steady states.

Emergence of objective, classical properties in quantum systems can be described in the modern language of quantum information theory. In this work, we present an example of such an analysis. We apply the quantum channel theory to a boson-spin model of open quantum systems and calculate, using recoilless approximation and the Floquet theory, the Holevo quantity, which bounds the capacity of the channel, broadcasting information about the central system into its environment. We analyze both the short-time regime, showing quadratic in time initial growth of the capacity, and the asymptotic regime. Complicated dependence on the model parameters, such as temperature, tunneling energy for the environment, etc., is also analyzed, showing, e.g., regimes where the Holevo bound reaches its maximum.

Understanding the thermodynamic properties of many-body quantum systems and their emergence from microscopic laws is a topic of great significance due to its profound fundamental implications and extensive practical applications. Recent advances in experimental techniques for controlling and preparing these systems have increased interest in this area, as they have the potential to drive the development of quantum technologies. In this study, we present a density-functional-theory approach to extract detailed information about the statistics of work and the irreversible entropy associated with quantum quenches at finite temperature. Specifically, we demonstrate that these quantities can be expressed as functionals of thermal and out-of-equilibrium densities, which may serve as fundamental variables for understanding finite-temperature many-body processes. We, then, apply our method to the case of the inhomogeneous Hubbard model, showing that our density-functional-theory-based approach can be usefully employed to unveil the distinctive roles of interaction and external potential on the thermodynamic properties of such a system.

We propose a hybrid-system quantum battery model composed of cavity quantum electrodynamics (QED) and optomechanics, which is called a cavity-optomechanical quantum battery (COQB). The single two-level system (TLS) serves as a quantum battery, which is influenced by a mechanical oscillator through a cavity mode. When the TLS is coupled with cavity mode in the Jaynes-Cummings (JC) model and the initial number of photons is 0, it is impossible to achieve the charging and discharging process. We exhibit three different methods to achieve the feasibility of the COQB and analyze the role of oscillator in the different scenarios. In the closed system, the cavity mode and TLS are coupled in the Rabi model. For the two different environments, the cavity mode is coupled with a thermal bath or driven by a coherent driving field. The existence of an oscillator in both the closed and open dynamics demonstrates advantages in the stored energy and ergotropy.

The exchange of information between an open quantum system and its environment, especially the backflow of information from the environment to the open system associated with quantum notions of non-Markovianity, is a widely discussed topic for years now. This information flow can be quantified by means of the trace distance of pairs of quantum states which provides a measure for the distinguishability of the states. The same idea can also be used to characterize the information flow in classical open systems through a suitable distance measure for their probability distributions on phase space. Here, we investigate the connection between the trace distance based quantum measure and the Kolmogorov distance for differently ordered quasiprobability distributions on phase space. In particular, we show that, for any pair of quantum states, one can find a unique quasiprobability distribution for which the Kolmogorov distance coincides with the trace distance. We further study the quantum-to-classical transition of the distance measures. By employing the Caldeira-Leggett model of quantum Brownian motion as a prototypical example, numerical simulations indicate a particularly rapid convergence of the Kolmogorov distance of the Wigner functions to the trace distance in the classical uncertainty limit, which establishes the Wigner function distance as an optimal tool for measuring semi-classical information backflow and for quantifying non-Markovianity in open continuous variable quantum systems.